Switchable volume hologram materials and devices

ABSTRACT

A new photopolymerizable material allows single-step, fast recording of volume holograms with properties that can be electrically controlled. Polymer-dispersed liquid crystals (PDLCs) in accordance with the invention preferably comprise a homogeneous mixture of a nematic liquid crystal and a multifunctional pentaacrylate monomer, in combination with photoinitiator, coinitiator and cross-linking agent. Optionally, a surfactant such as octanoic acid may also be added. The PDLC material is exposed to coherent light to produce an interference pattern inside the material. Photopolymerization of the new PDLC material produces a hologram of clearly separated liquid crystal domains and cured polymer domains. Volume transmission gratings made with the new PDLC material can be electrically switched between nearly 100% diffraction efficiency and nearly 0% diffraction efficiency. By increasing the frequency of the switching voltage, switching voltages in the range of 50 Vrms can be achieved. The optional use of surfactant allows low switching voltages at lower frequencies than without surfactant. In an alternative embodiment, a PDLC material in accordance with the invention can be utilized to form reflection gratings, including switchable reflection gratings. In still further embodiments, a PDLC material in accordance with the invention can be used to form switchable subwavelength gratings. By further processing, static transmission reflection, and subwavelength PDLC materials can be formed.

FIELD OF INVENTION

[0001] The present invention relates generally to photopolymerizablematerials, and more specifically to polymer-dispersed liquid crystalmaterials suitable for recording volume holograms.

BACKGROUND OF THE INVENTION

[0002] Typical state-of-the-art holographic materials do not have anelectro-optical nature which can be exploited for real time control oftheir optical properties. That is, once the hologram is fixed, itsoptical characteristics cannot be changed. Thus, it is seen that thereis a need for materials that can record volume holograms with propertiesthat can be electrically controlled.

[0003] Liquid crystals have long been utilized in the prior art fortheir ability to change their optical orientation in the presence of anelectric field. Additionally, liquid crystals can dramatically increasethe diffraction efficiency of a volume hologram of which they are apart. Together, these properties offer the very desirable possibility ofelectrically switching the diffraction efficiency of volume hologramsfor use in a wide variety of optical information processing and displayapplications.

[0004] The prior art has attempted to combine the properties of liquidcrystals with holograms by a variety of methods.

[0005] Unfortunately, most of these prior art devices are complex tomanufacture and are not successful at offering all the advantages ofvolume holographic gratings.

[0006] One approach for combining the advantages of liquid crystals withvolume holographic gratings has been to first make a holographictransmission grating by exposing a photopolymerizable material with aconventional two-beam apparatus for forming interference patterns insidethe material. After exposure, the material is processed to produce voidswhere the greatest amount of exposure occurred, that is, along thegrating lines, and then, in a further step, the pores are infused withliquid crystals. Unfortunately, these materials are complex tomanufacture and do not offer flexibility for in situ control over liquidcrystal domain size, shape, density, or ordering.

[0007] Polymer-dispersed liquid crystals (PDLCs) are formed from ahomogeneous mixture of prepolymer and liquid crystals. As the polymercures, the liquid crystals separate out as a distinct microdropletphase. If the polymer is a photopolymer, this phase separation occurs asthe prepolymer is irradiated with light. If a photopolymerizablepolymer-dispersed liquid crystal material is irradiated with light in asuitable pattern, a holographic transmission grating can be made insidethe cured polymer comprising gratings of cured polymer separated byphase separated liquid crystals. The prior art has attempted to employpolymer-dispersed liquid crystal materials for writing volume gratings,but, despite a variety of approaches, has not been able to achieve highefficiency in the Bragg regime, high density (small grating spacing)capability, or low voltage (<100 Vrms) switching for films in the rangeof 15 microns thickness. The inability to make an electricallyswitchable volume hologram that can be switched at voltages less than100 volts has been a particular deficiency in the prior art in thatlower voltages are necessary to be compatible with conventional displayand information processing technology.

OBJECTS OF THE INVENTION

[0008] It is, therefore, a principal object of the present invention toprovide an improved polymer-dispersed liquid crystal system suitable forrecording volume holograms.

[0009] It is a particular object of the present invention to provide apolymer-dispersed liquid crystal system that has a fast curing rate toproduce small liquid crystal droplets, particularly in the range of0.01-0.05 microns, for greater clarity of any resulting film and forwriting finer gratings.

[0010] It is another object of the present invention to provide asingle-step, fast holographic recording material.

[0011] It is a further object of the present invention to provideelectrically switchable volume holograms that can be switched atvoltages less than 100 volts.

[0012] It is also an object of the present invention to provide animproved polymer-dispersed liquid crystal system suitable for recordingreflection gratings, including, in particular, switchable reflectiongratings.

[0013] It is also an object of the present invention to provide animproved polymer-dispersed liquid crystal system suitable for recordingsubwavelength gratings, including, in particular switchablesubwavelength gratings.

[0014] These and other objects of the present invention will becomeapparent as the description of certain representative embodimentsproceeds.

SUMMARY OF THE INVENTION

[0015] The present invention provides a novel photopolymerizablematerial for single-step, fast recording of volume holograms withproperties that can be electrically controlled. The unique discovery ofthe present invention is a new homogeneous mixture of a nematic liquidcrystal and a multifunctional pentaacrylate monomer, with aphotoinitiator, a coinitiator and a cross-linking agent, thataccomplishes the objects of the invention, particularly the object offast curing speed and small liquid crystal droplet size.

[0016] Accordingly, the present invention is directed to apolymer-dispersed liquid crystal (“PDLC”) material, comprising themonomer dipentaerythritol hydroxypentaacrylate, a liquid crystal, across-linking monomer, a coinitiator and a photoinitiator dye. Thepolymer-dispersed liquid crystal material may optionally furthercomprise a surfactant. The PDLC material may be approximately 10-40 wt %of the liquid crystal. The PDLC material may be approximately 5-15 wt %of the cross-linking monomer. The amount of the coinitiator may be 10⁻³to 10⁻⁴ gram moles and the amount of the photoinitiator dye-may be 10⁻⁵to 10⁻⁶ gram moles. The surfactant, when present, may be up toapproximately 6 wt % of the PDLC material.

[0017] The present invention is also directed to an electricallyswitchable hologram, comprising a pair of transparent plates, andsandwiched between the transparent plates, a volume hologram made byexposing an interference pattern inside a polymer-dispersed liquidcrystal material, the polymer-dispersed liquid crystal materialcomprising, before exposure, the monomer dipentaerythritolhydroxypentaacrylate, a liquid crystal, a cross-linking monomer, acoinitiator and a photoinitiator dye. The electrically switchablehologram may optionally further comprise a surfactant.

[0018] The present Invention is additionally directed to a method forreducing the switching voltage needed to switch the optical orientationof liquid crystals in a polymer-dispersed liquid crystal material,comprising the step of using alternating current switching voltagefrequencies greater than 1000 Hz.

[0019] It is a feature of the present invention that a very clear andorderly separation of liquid crystal from cured polymer results, so asto produce high quality holographic transmission gratings. The prior arthas achieved generally only a distribution of large and small liquidcrystal domains and not the clear, orderly separation made possible bythe present invention.

[0020] It is also a feature of the present invention that volume Bragggratings with small grating spacings (approximately 4,000 lines per mm)can be recorded.

[0021] It is another feature of the present invention that in situcontrol of domain size, shape, density, and ordering is allowed.

[0022] It is yet another feature of the present invention that hologramscan be recorded using conventional optical equipment and techniques.

[0023] It is a further feature of the present invention that a uniquephotopolymerizable prepolymer material is employed. This unique materialcan be used to record holograms in a single step.

[0024] It is also a feature of the present invention that the PDLCmaterial has an anisotropic spatial distribution of phase-separatedliquid crystal droplets within a photochemically-cured polymer matrix.

[0025] It is an advantage of the present invention that single-steprecording is nearly immediate and requires no later development orfurther processing.

[0026] It is another advantage of the present invention that usesthereof are not limited to transmission gratings, but can be extended toother holograms such as optical storage devices and reflection andtransmission pictorial holograms.

[0027] It is also an advantage that, unlike holograms made withconventional photograph-type films or dichromated gels, holograms inaccordance with the present invention can be exposed in a one-stepprocess that requires little or no further processing.

[0028] It is a further advantage of the present invention thatreflection, transmission and pictorial holograms made using theteachings provided herein can be switched on and off.

[0029] It is also an advantage of the present invention that switchablereflection gratings can be formed using positive and negative dielectricanisotropy liquid crystals. These and other features and advantages ofthe present invention will become apparent as the description of certainrepresentative embodiments proceeds.

BRIEF DESCRIPTION OF FIGURES

[0030] The present invention will be more clearly understood from areading of the following detailed description in conjunction with theaccompanying figures wherein:

[0031]FIG. 1 is a cross-sectional view of an electrically switchablehologram made of an exposed polymer-dispersed liquid crystal materialaccording to the teachings of the present invention;

[0032]FIG. 2 is a graph of the normalized net transmittance andnormalized net diffraction efficiency of a hologram made according tothe teachings of the present invention (without the addition of asurfactant) versus the rms voltage applied across the hologram;

[0033]FIG. 3 is a graph of both the threshold and complete switching rmsvoltages needed for switching a hologram made according to the teachingsof the present invention -to minimum diffraction efficiency versus thefrequency of the rms voltage;

[0034]FIG. 4 is a graph of the normalized diffraction efficiency as afunction of the applied electric field for a PDLC material formed with34% by weight liquid crystal without surfactant present and a PDLCmaterial formed-with 29% by weight liquid crystal and 4% by weightsurfactant;

[0035]FIG. 5 is a graph showing the switching response time data for thediffracted beam in the surfactant-containing PDLC material in FIG. 5;

[0036]FIG. 6 is a graph of the normalized net transmittance and thenormalized net diffraction efficiency of a hologram made according tothe teachings of the present invention versus temperature;

[0037]FIG. 7 is an elevational view of a typical experimentalarrangement for recording reflection gratings;

[0038]FIGS. 8a and 8 b are elevational views of a reflection grating inaccordance with the present invention having periodic planes of polymerchannels and PDLC channels disposed parallel to the front surface in theabsence of a field (FIG. 8a) and with an electric field applied (FIG.8b) wherein the liquid-crystal utilized in the formation of the gratinghas a positive dielectric anisotropy;

[0039]FIGS. 9a and 9 b are elevational views of a reflection grating inaccordance with the invention having periodic planes of polymer channelsand PDLC channels disposed parallel to the front surface of the gratingin the absence of an electric field (FIG. 9a) and with an electric fieldapplied (FIG. 9b) wherein the liquid crystal utilized in the formationof the grating has a negative dielectric anisotropy;

[0040]FIG. 10a is an elevational view of a reflection grating inaccordance with the invention disposed within a magnetic field generatedby Helmholtz coils;

[0041]FIGS. 10b and 10 c are elevational views of the reflection gratingof FIG. 10a in the absence of an electric field (FIG. 10b) and with anelectric field applied (FIG. 10c);

[0042]FIGS. 11a and 11 b are representative side views of a slantedtransmission grating (FIG. 11a) and a slanted reflection grating (FIG.11b) showing the orientation of the grating vector G of the periodicplanes of polymer channels and PDLC channels;

[0043]FIG. 12 is an elevational view of a reflection grating formed inaccordance with the invention while a shear stress field is applied;

[0044]FIG. 13 is an elevational view of a subwavelength grating inaccordance with the present invention having periodic planes of polymerchannels and PDLC channels disposed perpendicular to the front surfaceof the grating;

[0045]FIG. 14a is an elevational view of a switchable subwavelengthgrating in accordance with the present invention wherein thesubwavelength grating functions as a half wave plate whereby thepolarization of the incident radiation is rotated by 90°;

[0046]FIG. 14b is an elevational view of the switchable half wave plateshown in FIG. 14a disposed between crossed polarizers whereby theincident light is transmitted;

[0047]FIGS. 14c and 14 d are side views of the switchable half waveplate and crossed polarizers shown in FIG. 14b showing the effect of theapplication of a voltage to the plate whereby the polarization of thelight is no longer rotated and thus blocked by the second polarizer;

[0048]FIG. 15a is a side view of a switchable subwavelength grating inaccordance with the invention wherein the subwavelength gratingfunctions as a quarter wave plate whereby plane polarized light istransmitted through the subwavelength grating, retroreflected by amirror and reflected by the beam splitter;

[0049]FIG. 15b is a side view of the switchable subwavelength grating ofFIG. 15a showing the effect of the application of a voltage to the platewhereby the polarization of the light is no longer modified, therebypermitting the reflected light to pass through the beam splitter;

[0050]FIGS. 16a and 16 b are elevational views of a subwavelengthgrating in accordance with the present invention having periodic planesof polymer channels and PDLC channels disposed perpendicular to thefront face of the grating in the absence of an electric field (FIG. 16a)and with an electric field applied (FIG. 16b) wherein the liquid crystalutilized in formation of the grating has a positive dielectricanisotropy; and

[0051]FIG. 17 is a side view of five subwavelength gratings wherein thegratings are stacked and connected electrically in parallel therebyreducing the switching voltage of the subwavelength grating.

DETAILED DESCRIPTION

[0052] In accordance with present invention there is provided a polymerdispersed liquid crystal (“PDLC”) material comprising a monomer, adispersed liquid crystal, a crosslinking monomer, a coinitiator and aphotoinitiator dye. These PDLC materials exhibit clear and orderlyseparation of the liquid crystal and cured polymer, whereby the PDLCmaterial advantageously provides high quality holographic gratings. ThePDLC materials of the present invention are also advantageously formedin a single step. The present invention also utilizes a uniquephotopolymerizable prepolymer material that permits in situ control overcharacteristics of the resulting gratings, such as domain size, shape,density, ordering, and the like. Furthermore, methods and materials ofthe present invention can be used to prepare PDLC materials thatfunction as switchable transmission or reflection gratings.

[0053] Polymer dispersed liquid crystal materials, methods, and devicescontemplated for use in the practice of the present invention are alsodescribed in R. L. Sutherland et al., “Bragg Gratings in an AcrylatePolymer Consisting of Periodic Polymer-Dispersed Liquid-Crystal Planes,”Chemistry of Materials, No. 5, pp. 1533-1538 (1993); in R. L. Sutherlandet al., “Electrically switchable volume gratings in polymer-dispersedliquid crystals,” Applied Physics Letters, Vol. 64, No. 9, pp. 1074-1076(1984); and T. J. Bunning et al., “The Morphology and Performance ofHolographic Transmission Gratings Recorded in Polymer Dispersed LiquidCrystals,” Polymer, Vol. 36, No. 14, pp. 2699-2708 (1995), all of whichare fully incorporated by reference into this Detailed Description.Copending patent applications Ser. Nos. 08/273,436 and 08/273,437Sutherland et al., titled “Switchable Volume Hologram Materials andDevices,” and “Laser Wavelength Detection and Energy Dosimetry Badge,”respectively, also incorporated herein by reference include backgroundmaterial on the formation of transmission gratings inside volumeholograms.

[0054] The process by which a hologram is formed according to theinvention is controlled primarily by the choice of components used toprepare the homogeneous starting mixture, and to a lesser extent by theintensity of the incident light pattern. The preferred polymer-dispersedliquid crystal (“PDLC”) material employed in the practice of the presentinvention creates a switchable hologram in a single step. A new featureof the preferred PDLC material is that illumination by an inhomogeneous,coherent light pattern initiates a patterned, anisotropic diffusion (orcounter diffusion) of polymerizable monomer and second phase material,particularly liquid crystal (“LC”) for this application. Thus,alternating well-defined channels of second phase-rich material,separated by well-defined channels of nearly pure polymer, are producedin a single-step process.

[0055] The resulting preferred PDLC material has an anisotropic spatialdistribution of phase-separated LC droplets within the photochemicallycured polymer matrix. Prior art PDLC materials made by a single-stepprocess can achieve at best only regions of larger LC bubbles andsmaller LC bubbles in a polymer matrix. The large bubble sizes arehighly scattering which produces a hazy appearance and multiple orderdiffractions, in coontrast to the well-defined first order diffractionand zero order diffraction made possible by the small LC bubbles of thepreferred PDLC material in well-defined channels of LC-rich material.Reasonably well-defined alternately LC-rich channels and nearly purepolymer channels in a PDLC material are possible by multi-stepprocesses, but such processes do not achieve the precise morphologycontrol over LC droplet size and distribution of sizes and widths of thepolymer and LC-rich channels made possible by the preferred PDLCmaterial.

[0056] The sample is prepared by coating the mixture between twoindium-tin-oxide (ITO) coated glass slides separated by spacers ofnominally 10-20 μm thickness. The sample is placed in a conventionalholographic recording-setup. Gratings are typically recorded using the488 nm line of an Argon ion laser with intensities of between about0.1-100 mW/cm² and typical exposure times of 30-120 seconds. The anglebetween the two beams is varied to vary the spacing of the intensitypeaks, and hence the resulting grating spacing of the hologram.Photopolymerization is induced by the optical intensity pattern. A moredetailed discussion of exemplary recording apparatus can be found in R.L. Sutherland, et al., “Switchable holograms in new photopolymer-liquidcrystal composite materials,” Society of Photo-Optical InstrumentationEngineers (SPIE), Proceedings Reprint, Volume 2404, reprinted fromDiffractive and Holographic Optics Technology II (1995), incorporatedherein by reference.

[0057] The features of the PDLC material are influenced by thecomponents used in the preparation of the homogeneous starting mixtureand, to a lesser extent, by the intensity of the incident light pattern.In the preferred embodiment, the prepolymer material comprises a mixtureof a photopolymerizable monomer, a second phase material, aphotoinitiator dye, a coinitiator, a chain extender (or cross-linker),and, optionally, a surfactant.

[0058] In the preferred embodiment, the two major components of theprepolymer mixture are the polymerizable monomer and the second phasematerial, which are preferably completely miscible. Highlyfunctionalized monomers are preferred because they form denselycross-linked networks which shrink to some extent and tend to squeezeout the second phase material. As a result, the second phase material ismoved anisotropically out of the polymer region and, thereby, separatedinto well-defined polymer-poor, second phase-rich regions or domains.Highly functionalized monomers are also preferred because the extensivecross-linking associated with such monomers yields fast kinetics,allowing the hologram to form relatively quickly, whereby the secondphase material will exist in domains of less than approximately 0.1 μm.

[0059] Highly functionalized monomers, however, are relatively viscous.As a result, these monomers do not tend to mix well with othermaterials, and they are difficult to spread into thin films.Accordingly, it is preferable to utilize a mixture of pentaacrylates incombination with di-, tri-, and/or tetra-acrylates in order to optimizeboth the functionality and viscosity of the prepolymer material.Suitable acrylates, such as triethyleneglycol diacrylate,trimethylolpropane triacrylate, pentaerythritol triacrylate,pentaerythritol tetracrylate, pentaerythritol pentacrylate, and the likecan be used in accordance with the present invention. In the preferredembodiment, it has been found that an approximately 1:4 mixture of tri-to penta-acrylate facilitates homogeneous mixing while providing afavorable mixture for forming 10-20 μm films on the optical plates.

[0060] The second phase material of choice for use in the practice ofthe present invention is a liquid crystal. This also allows anelectro-optical response for the resulting hologram. The concentrationof LC employed should be large enough to allow a significant phaseseparation to occur in the cured sample, but not so large as to make thesample opaque or very hazy. Below about 20% by weight very little phaseseparation occurs and diffraction efficiencies are low. Above about 35%by weight, the sample becomes-highly scattering, reducing bothdiffraction efficiency and transmission. Samples fabricated withapproximately 25% by weight typically yield good diffraction efficiencyand optical clarity. In prepolymer mixtures utilizing a surfactant, theconcentration of LC may be increased to 35% by weight without loss inoptical performance by adjusting the quantity of surfactant. Suitableliquid crystals contemplated for use in the practice of the presentinvention include the mixture of cyanobiphenyls marketed as E7 by Merck,4′-n-pentyl-4-cyanobiphenyl, 4′-n-heptyl-4-cyanobiphenyl,4′-octaoxy-4-cyano biphenyl, 4′-pentyl-4-cyanoterphenyl,∝-methoxybenzylidene-4′-butylaniline, and the like. Other second phasecomponents are also-possible.

[0061] The preferred polymer-dispersed liquid crystal material employedin the practice of the present invention is formed from a prepolymermaterial that is a homogeneous mixture of a polymerizable monomercomprising dipentaerythritol hydroxypentacrylate (available, forexample, from Polysciences, Inc., Warrington, Pa.), approximately 10-40wt % of the liquid crystal E7 (which is a mixture of cyanobiphenylsmarketed as E7 by Merck and also available from BDH Chemicals, Ltd.,London, England), the chain-extending monomer N-vinylpryrrolidone(“NVP”)(available from the Aldrich Chemical Company, Milwaukee, Wis.),coinitiator N-phenylgylycine (“NPG”) (also available from the AldrichChemical Company, Milwaukee, Wis.), and the photoinitiator dye rosebengal ester;(2,4,5,7-tetraiodo-3′,4′,5′,6′-tetrachlorofluroescein-6-acetate ester)marketed as RBAX by Spectragraph, Ltd., Maumee, Ohio). Rose bengal isalso available as rose bengal sodium salt (which must be esterfied forsolubility) from the Aldrich Chemical Company. This system has a veryfast curing speed which results in the formation of small liquid crystalmicro-droplets.

[0062] The mixture of liquid crystal and prepolymer material arehomogenized to a viscous solution by suitable means (e.g.,ultrasonification) and spread between indium-tin-oxide (“ITO”) coatedglass slides with spacers of nominally 15-100 μm thickness and,preferably, 10-20 μm thickness. The ITO is electrically conductive andserves as an optically transparent electrode. Preparation, mixing andtransfer of the prepolymer material onto the glass slides are preferablydone in the dark as the mixture is extremely sensitive to light.

[0063] The sensitivity of the prepolymer materials to light intensity isdependent on the photoinitiator dye and its concentration. A higher dyeconcentration leads to a higher sensitivity. In most cases, however, thesolubility of the photoinitiator dye limits the concentration of the dyeand, thus, the sensitivity of the prepolymer material. Nevertheless ithas been found that for more general applications photoinitiator dyeconcentrations in the range of 0.2-0.4% by weight are sufficient toachieve desirable sensitivities and allow for a complete bleaching ofthe dye in the recording process, resulting in colorless final samples.Photoinitiator dyes that are useful in generating PDLC materials inaccordance with the present invention are rose bengal ester(2,4,5,7-tetraiodo-3′,4′,5′,6′-tetrachlorofluroescein-6-acetate ester);rose bengal sodium salt; rosin; eosin sodium salt; 4,5-diiodosuccinylfluorescein; camphorquinone; methylene blue, and the like. These dyesallow a sensitivity to recording wavelengths across the visible spectrumfrom nominally 400 nm to 700 nm. Suitable near-infrared dyes, such ascationic cyanine dyes with trialkylborate anions having absorption from600-900 nm as well as merocyanine dyes derived from spiropyran shouldalso find utility in connection with the present invention.

[0064] The coinitiator employed in the practice of the present inventioncontrols the rate of curing in the free radical polymerization reactionof the prepolymer material optimum phase separation and, thus, optimumdiffraction efficiency in the resulting PDLC material, are a function ofcuring rate. It has been found that favorable results can be achievedutilizing coinitiator in the range of 2-3% by weight. Suitablecoinitiators include N-phenyl glycine; triethylene amine;triethanolamine; N,N-dimethyl-2,6-diisopropyl aniline, and the like.

[0065] Other suitable dyes and dye coinitiator combinations that shouldbe suitable for use in the present invention, particularly for visiblelight, include eosin and triethanolamine; camphorquinone andN-phenyglycine; fluorescein and triethanolamine; methylene blue andtriethanolamine or N-phenylglycine; erythrosin B and triethanolamine;indolinocarbocyanine and triphenyl borate; iodobenzospiropyran andtriethylamine, and the like.

[0066] The chain extender (or cross linker) employed in the practice ofthe present invention helps to increase the solubility of the componentsin the prepolymer material as well as increase the speed ofpolymerization. The chain extender is preferably a smaller vinyl monomeras compared with the pentacrylate, whereby it can react with theaerylate positions in the pentaacrylate monomer, which are not easilyaccessible to neighboring pentaacrylate monomers due to sterichindrance. Thus, reaction of the chain extender monomer with the polymerincreases the propagation length of the growing polymer and results inhigh molecular weights. It has been found that chain extender in generalapplications in the range of 10-18% by weight maximizes the performancein terms of diffraction efficiency. In the preferred embodiment, it isexpected that suitable chain extenders can be selected from thefollowing: N-vinyl pyrrolidone; N-vinyl pyridine; acrylonitrile; N vinylcarbazole, and the like.

[0067] It has been found that the addition of a surfactant material,namely, octanoic acid, in the prepolymer material lowers the switchingvoltage and also improves the diffraction efficiency. In particular, theswitching voltage for PDLC materials containing a surfactant aresignificantly lower than those of a PDLC material made without thesurfactant. While not wishing to be bound by any particular theory, itis believed that these results may be attributed to the weakening of theanchoring forces between the polymer and the phase-separated LCdroplets. SEM studies have shown that droplet sizes in PDLC materialsincluding surfactants are reduced to the range of 30-50 nm and thedistribution is more homogeneous. Random scattering in such materials isreduced due to the dominance of smaller droplets, thereby increasing thediffraction efficiency. Thus, it is believed that the shape of thedroplets becomes more spherical in the presence of surfactant, therebycontributing to the decrease in switching voltage.

[0068] For more general applications, it has been found that sampleswith as low as 5% by weight of surfactant exhibit a significantreduction in switching voltage. It has also been found that, whenoptimizing for low switching voltages, the concentration of surfactantmay vary up to about 10% by weight (mostly dependent on LCconcentration) after which there is a large decrease in diffractionefficiency, as well as an increase in switching voltage (possibly due toa reduction in total phase separation of LC). Suitable surfactantsinclude octanoic acid; heptanoic acid; hexanoic acid; dodecanoic acid;decanoic acid, and the like.

[0069] In samples utilizing octanoic acid as the-surfactant, it has beenobserved that the conductivity of the sample is high, presumably owingto the presence of the free carboxyl (COOH) group in the octanoic acid.As a result, the sample increases in temperature when a high frequency(−2 KHz) electrical field is applied for prolonged periods of time.Thus, it is desirable to reduce the high conductivity introduced by thesurfactant, without sacrificing the high diffraction efficiency and thelow switching voltages. It has been found that suitable electricallyswitchable gratings can be formed from a polymerizable monomer, vinylneononanoate (“VN”) C₈H₁₇C₂CH═CH₂, commercially available from theAldrich Chemical Co. in Milwaukee, Wis. Favorable results have also beenobtained where the chain extender N-vinyl pyrrolidone (“NVP”) and thesurfactant octanoic acid are replaced by 6.5% by weight VN. VN also actsas a chain extender due to the presence of the reactive acrylate monomergroup. In these variations, high optical quality samples were obtainedwith about 70% diffraction efficiency, and the resulting gratings couldbe electrically switched by an applied field of 6V/μm.

[0070] PDLC materials in accordance with the present invention may alsobe formed using a liquid crystalline bifunctional acrylate as themonomer (“LC monomer”). The LC monomers have an advantage overconventional acrylate monomers due to their high compatibility with thelow molecular weight nematic LC materials, thereby facilitatingformation of high concentrations of low molecular weight LC and yieldinga sample with high optical quality. The presence of higherconcentrations of low molecular weight LCs in the PDLC material greatlylowers the switching voltages (e.g., to ˜2V/μm). Another advantage ofusing LC monomers is that it is possible to apply low AC or DC fieldswhile recording holograms to pre-align the host LC monomers and lowmolecular weight LC so that a desired orientation and configuration ofthe nematic directors can be obtained in the LC droplets. The chemicalformulae of several suitable LC monomers are as follows:

CH₂═CH—COO—(_(CH2))₆O—C₆H₅—-C₆H₅—COO—CH═CH₂

CH₂═CH—(CH₂)₈—COO—C₆H₅—COO—(CH₂)₈—CH═CH₂

H(CF₂)₁₀CH₂O—CH₂—C(═CH₂)—COO—(CH₂CH₂O)₃CH₂CH₂O—COO—CH₂—C(═CH₂)—CH₂O(CF₂)₁₀H

[0071] Semifluorinated polymers are known to show weaker anchoringproperties and also significantly reduced switching fields. Thus, itis-believed that semifluorinated acrylate monomers which arebifunctional and liquid crystalline will find suitable application inthe present invention.

[0072] Referring now to FIG. 1 of the drawings, there is shown across-sectional view of an electrically switchable hologram 10 made ofan exposed polymer-dispersed liquid crystal material according to theteachings of the present invention. A layer 12 of the polymer-dispersedliquid crystal material is sandwiched between a pair of indium-tin-oxide(ITO) coated glass slides 14 and spacers 16. The interior of hologram 10shows the Bragg transmission gratings 18 formed when layer 12 wasexposed to an interference pattern from two intersecting beams ofcoherent laser light. The exposure times and intensities can be varieddepending on the diffraction efficiency and liquid crystal domain sizedesired. Liquid crystal domain size can be controlled by varying theconcentrations of photoinitiator, coinitiator and chain-extending (orcross-linking) agent. The orientation of the nematic directors can becontrolled while the gratings are being recorded by application of anexternal electric field across the ITO electrodes.

[0073] The scanning electron micrograph shown in FIG. 2 of thereferenced Applied Physics Letters article and incorporated herein byreference is of the surface of a grating which was recorded in a samplewith a 36 wt % loading of liquid crystal using the 488 nm line of anargon ion laser at an intensity of 95 mW/cm². The size of the liquidcrystal domains is about 0.2 μm and the grating spacing is about 0.54μm. This sample, which is approximately 20 μm thick, diffracts light inthe Bragg regime.

[0074]FIG. 2 is a graph of the normalized net transmittance andnormalized net diffraction efficiency of a hologram made according tothe teachings of the present invention-versus the root mean squarevoltage (“Vrms”) applied across the hologram. Δη is the change in firstorder Bragg diffraction efficiency. ΔT is the change in zero ordertransmittance. FIG. 2 shows that energy is transferred from the firstorder beam to the zero-order beam as the voltage is increased. There isa true minimum of the diffraction efficiency at approximately 225 Vrms.The peak diffraction efficiency can approach 100%, depending on thewavelength and polarization of the probe beam, by appropriate adjustmentof the sample thickness. The minimum diffraction efficiency can be madeto approach 0% by slight adjustment of the parameters of the PDLCmaterial to force the refractive index of the cured polymer to be equalto the ordinary refractive index of the liquid crystal.

[0075] By increasing the frequency of the applied voltage, the switchingvoltage for minimnum diffraction efficiency can be decreasedsignificantly. This is illustrated in FIG. 3, which is a graph of boththe threshold rms voltage 20 and the complete switching rms voltage 22needed for switching a hologram made according to the teachings of thepresent invention to minimum diffraction efficiency versus the frequencyof the rms voltage. The threshold and complete switching rms voltagesare reduced to 20 Vrms and 60 Vrms, respectively, at 10 kHz. Lowervalues are expected at even higher frequencies.

[0076] Smaller liquid crystal droplet sizes have the problem that ittakes high switching voltages to switch their orientation. As describedin the previous paragraph, using alternating current switching voltagesat high frequencies helps reduce the needed switching voltage. Asdemonstrated in FIG. 4, another unique discovery of the presentinvention is that adding a surfactant (e.g., octanoic acid) to theprepolymer material in amounts of about 4%-6% by weight of the totalmixture resulted in sample holograms with switching voltages near 50Vrms at lower frequencies of 1-2 kHz. As shown in FIG. 5, it has alsobeen found that the use of the surfactant with the associated reductionin droplet size, reduces the switching time of the PDLC materials. Thus,samples made with surfactant can be switched on the order of 25-44microseconds. Without wishing to be bound by any theory, the surfactantis believed to reduce switching voltages by reducing the anchoring ofthe liquid crystals at the interface between liquid crystal and curedpolymer.

[0077] Thermal control of diffraction efficiency is illustrated in FIG.6. FIG. 6 is a graph of the normalized net transmittance and normalizednet diffraction efficiency of a hologram made according to the teachingsof the present invention versus temperature.

[0078] The polymer-dispersed liquid crystal materials described hereinsuccessfully demonstrate the utility for recording volume holograms of aparticular composition for such polymer-dispersed liquid crystalsystems. Although the disclosed polymer-dispersed liquid crystal systemsare specialized, the present invention will find application in otherareas where a fast curing polymer and a material that can bephase-separated from the polymer will find use.

[0079] As shown in FIG. 7, a PDLC reflection grating is prepared byplacing several drops of the mixture of prepolymer material 112 on anindium-tin oxide (“ITO”)-coated glass slide 114 a. A second indium-tinoxide (“ITO”) coated slide 114 b is then pressed against the first,thereby causing the prepolymer material 112 to fill the region betweenthe slides 114 a and 114 b. Preferably, the separation of the slides ismaintained at approximately 20 μm by utilizing uniform spacers 118.Preparation, mixing and transfer of the prepolymer material ispreferably done in the dark. Once assembled, a mirror 116 is placeddirectly behind the glass plate 114 b. The distance of the mirror fromthe sample is preferably substantially shorter than the coherence lengthof the laser. The PDLC material is preferably exposed to the 488 nm lineof an argon-ion laser, expanded to fill the entire plane of the glassplate, with an intensity of approximately 0.1-100 mWatts/cm² withtypical exposure times is of 30-120 seconds. Constructive anddestructive interference within the expanded beam establishes a periodicintensity profile through the thickness of the film.

[0080] In the preferred embodiment, the prepolymer material utilized tomake a reflection grating comprises a monomer, a liquid crystal, across-linking monomer, a coinitiator, and a photoinitiator dye. In thepreferred embodiment, the reflection grating is formed from prepolymermaterial comprising by total weight of the monomer dipentaerythritolhydroxypentacrylate (“DPHA”), 34% by total weight of a liquid crystalcomprising a mixture of cyano biphenyls. (known commercially as “E7”),10% by total weight of a cross-linking monomer comprisingN-vinylpyrrolidone (“NVP”), 2.5% by weight of the coinitiatorN-phenylglycine (“NPG”), and 10⁻⁵ to 10⁻⁶ gram moles of a photoinitiatordye comprising rose bengal ester. Further, as with transmissiongratings, the addition of surfactants is expected to facilitate the sameadvantageous properties discussed above in connection with transmissiongratings. It is also expected that similar ranges and variation ofprepolymer starting materials will find ready application in theformation of suitable reflection gratings.

[0081] It has been determined by low voltage, high resolution scanningelectron microscopy (“LVHRSEM”) that the resulting material comprises afine grating with a periodicity of 165 nm with the grating vectorperpendicular to the plane of the surface. Thus, as shown schematicallyin FIG. 8a, grating 130 includes periodic planes of polymer channels 130a and PDLC channels 130 b which run parallel to the front surface 134.The grating spacing associated with these periodic planes remainsrelatively constant throughout the full thickness of the sample from theair/film to the film/substrate interface.

[0082] Although interference is used to prepare both transmission andreflection gratings, the morphology of the reflection grating differssignificantly. In particular, it has been determined that, unliketransmission gratings with similar liquid crystal concentrations, verylittle coalescence of individual droplets was evident. Furthermore, thedroplets that were present in the material were significantly smaller,having diameters between 50 and 100 nm. Furthermore, unlike transmissiongratings where the liquid crystal-rich regions typically comprise lessthan 40% of the grating, the liquid crystal-rich component of areflection grating is significantly larger. Due to the much smallerperiodicity associated with reflection gratings, i.e., a narrowergrating spacing (˜0.2 microns), it is believed that the time differencebetween completion of curing in high intensity versus low intensityregions is much smaller. Thus, gelation occurs more quickly and dropletgrowth is minimized. It is also believed that the fast polymerization,as evidenced by small droplet diameters, traps a significant percentageof the liquid crystal in the matrix during gelation and precludes anysubstantial growth of large droplets or diffusion of small droplets intolarger domains.

[0083] Analysis of the reflection notch in the absorbance spectrumsupports the conclusion that a periodic refractive index modulation isdisposed through the thickness 6% the film. In PDLC materials that areformed with the 488 nm line of an argon ion laser, the reflection notchtypically has a reflection wavelength at approximately 472 nm for normalincidence and a relatively narrow bandwidth. The small differencebetween the writing wavelength and the reflection wavelength(approximately 5%) indicates that shrinkage of the film is not asignificant problem. Moreover, it has been found that the performance ofsuch gratings is stable over periods of many months.

[0084] In addition to the materials utilized in the preferred embodimentdescribed above, it is believed that suitable PDLC materials could beprepared utilizing monomers such as triethyleneglycol diacrylate,trimethylolpropane-triacrylate, pentaerythritol triacrylate,pentaerythritol tetracrylate, pentaerythritol pentacrylate, and thelike. Similarly, other coinitiators such as triethylamine,triethanolamine, N,N-dimethyl-2,6-diisopropylaniline, and the like couldbe used instead of N-phenyl glycine. Where it is desirable-to use the458 nm, 476 nm, 488 nm or 514 nm lines of an Argon lon laser, that thephotoinitiator dyes rose bengal sodium salt, eosin, eosin sodium salt,fluorescein sodium salt and the like will give favorable results. Wherethe 633 nm line is utilized, methylene blue will find ready application.Finally, it is believed that other liquid crystals, such as4′-pentyl-4-cyanobiphenyl or 4′-heptyl-4-cyanobiphenyl, can be utilizedin accordance with the invention.

[0085] Referring again to FIG. 8a, there is shown an elevational view ofa reflection grating 130 in accordance with the invention havingperiodic planes of polymer channels 130 a and PDLC channels 130 bdisposed parallel to the front surface 134 of the grating 130. Thesymmetry axis 136 of the liquid crystal domains is formed in a directionperpendicular to the periodic channels 130 a and 130 b of the grating130 and perpendicular to the front surface 134 of the grating 130. Thus,when an electric field E is applied, as shown in FIG. 8b, the symmetryaxis 136 is already in a low energy state in alignment with the field Eand will not reorient. Thus, reflection gratings formed in accordancewith the procedure described above will not normally be switchable.

[0086] In general, a reflection grating tends to reflect a narrowwavelength band, such that the grating can be used as a reflectionfilter. In the preferred embodiment, however, the reflection grating isformed so that it will be switchable. In accordance with the presentinvention, switchable reflection gratings can be made utilizing negativedielectric anisotropy LCs (or LCs with a low cross-over frequency), anapplied magnetic field, an applied shear stress field, or slantedgratings.

[0087] It is known that liquid crystals having a negative dielectricanisotropy (Δε) will rotate in a direction perpendicular to an appliedfield. As shown in FIG. 9a, the symmetry axis 136 of the liquid crystaldomains formed with a liquid crystal having a negative Δε will also bedisposed in a direction perpendicular to the periodic channels 130 a and130 b of the grating 130 and to the front surface 134 of the grating.However, when an electric field E is applied across such gratings, asshown in FIG. 9b, the symmetry axis of the negative Δε liquid crystalwill distort and reorient in a direction perpendicular to the field E,which is perpendicular to the film and the periodic planes of thegrating. As a result, the reflection grating can be switched between astate where it is reflective and a state where it is transmissive. Thefollowing negative Δε liquid crystals and others are expected to findready application in the methods and devices of the present invention:

[0088] Liquid crystals can be found in nature (or synthesized) witheither positive or negative Δε. Thus, in more detailed aspects of theinvention, it is possible to use a LC which has a positive Δε at lowfrequencies, but becomes negative at high frequencies. The frequency (ofthe applied voltage) at which Δε changes sign is called the cross-overfrequency. The cross-over frequency will vary with LC composition, andtypical values range from 1-10 kHz. Thus, by operating at the properfrequency, the reflection grating may be switched. In accordance withthe invention, it is expected that low crossover frequency materials canbe prepared from a combination of positive and negative dielectricanisotropy liquid crystals. A suitable positive dielectric liquidcrystal for use in such a combination contains four ring esters as shownbelow:

[0089] A strongly negative dielectric liquid crystal suitable for use insuch a combination is made up of pyridazines as shown below:

[0090] Both liquid crystal materials are available from LaRoche & Co.,Switzerland. By varying the proportion of the positive and negativeliquid crystals in the combination, crossover frequencies from 1.4-2.3kHz are obtained at room temperature. Another combination suitable foruse in the present embodiment is a combination of the following:p-pentylphenyl-2-chloro-4-(p-pentylbenzoyloxy) benzoate and-4-(p-pentylbenzoyloxy) benzoate andp-heptylphenyl-2-chloro-4-(p-octylbenzoyloxy) benzoate. These materialsare available from Kodak Company.

[0091] In still more detailed aspects of the invention, switchablereflection gratings can be formed using positive are liquid crystals. Asshown in FIG. 10a, such gratings are formed by exposing the PDLCstarting material to a magnetic field during the curing process. Themagnetic field can be generated by the use of Helmholtz coils (as shownin FIG. 10a), the use of a permanent magnet, or other suitable means.Preferably, the magnetic field M is oriented parallel to the frontsurface of the glass plates (not shown) that are used to form thegrating 140. As a result, the symmetry axis 146 of the liquid crystalswill orient along the field while the mixture is fluid. Whenpolymerization is complete, the field may be removed and the alignmentof the symmetry axis of the liquid crystals will remain unchanged. (SeeFIG. 10b.) When an electric field is applied, as shown in FIG. 10c, thepositive Δε liquid crystal will reorient in the direction of the field,which is perpendicular to the front surface of grating and to theperiodic channels of the grating.

[0092]FIG. 11a depicts a slanted transmission grating 148 and FIG. 11bdepicts a slanted reflection grating 150. A holographic transmissiongrating is considered slanted if the direction of the grating vector Gis not parallel to the grating surface. In a holographic reflectiongrating, the grating is said to be slanted if the grating vector G isnot perpendicular to the grating surface. Slanted gratings have many ofthe same uses as subslanted gratings such as visual displays, mirrors,line filters, optical switches, and the like.

[0093] Primarily, slanted holographic gratings are used to control thedirection of a diffracted beam. For example, in reflection holograms aslanted grating is used to separate the specular reflection of the filmfrom the diffracted beam. In a PDLC holographic grating, a slantedgrating has an even more useful advantage. The slant allows themodulation depth of the grating to be controlled by an electric fieldwhen using either tangential or homeotropic aligned liquid crystals.This is because the slant provides components of the electric field inthe directions both tangent and perpendicular to the grating vector. Inparticular, for the reflection grating, the LC domain symmetry axis willbe oriented along the grating vector G and can be switched to adirection perpendicular to the film plane by a longitudinally appliedfield E. This is the typical geometry for switching of the diffractionefficiency of a slanted reflection grating.

[0094] When recording slanted reflection gratings, it is desirable toplace the sample between the hypotenuses of two right-angle glassprisms. Neutral density filters can then be placed in optical contactwith the back faces of the prisms using index matching fluids so as tofrustrate back reflections which would cause spurious gratings to alsobe recorded. The incident laser beam is split by a conventional beamsplitter into two beams which are then directed to the front faces ofthe prisms, and then overlapped in the sample at the desired angle. Thebeams thus enter the sample from opposite sides. This prism couplingtechnique permits the light to enter the sample at greater angles. Theslant of the resulting grating is determined by the angle which theprism assembly is rotated (i.e., the angle between the direction of oneincident beam and the normal to the prism front face at which that beamenters the prism).

[0095] As shown in FIG. 12, switchable reflection gratings may be formedin the presence of an applied shear stress field. In this method, ashear stress would be applied along the direction of a magnetic field M.This could be accomplished, for example, by applying equal and oppositetensions to the two ITO coated glass plates which sandwich theprepolymer mixture while the polymer is still soft. This shears-stresswould distort the LC domains in the direction of the stress, and theresultant LC domain symmetry axis will be preferentially along thedirection of the stress,.parallel to the PDLC planes and perpendicularto the direction of the applied electric field for switching.

[0096] Reflection gratings prepared in accordance with the teachings ofthe present invention will find application in color reflectivedisplays, switchable wavelength filters for laser protection, and thelike.

[0097] In another embodiment of the present invention, PDLC materialscan be made that exhibit a property known as form birefringence wherebypolarized light that is transmitted through the grating will have itspolarization modified. Such gratings are known as subwavelengthgratings, and they behave like a negative uniaxial crystal, such ascalcite, potassium dihydrogen phosphate, or lithium niobate, with anoptic axis perpendicular to the PDLC planes. Referring now to FIG. 13,there is shown an elevational view of a transmission grating 200 inaccordance with the present invention having periodic planes of polymerchannels 200 a and PDLC channels 200 b disposed perpendicular to thefront surface 204 of the grating 200. The optic axis 206 is disposedperpendicular to polymer planes 200 a and the PDLC planes 200 b. Eachpolymer plane 200 a has a thickness t_(p) and refractive index

and each PDLC plane 200 b has a thickness t_(PDLC) and refractive indexn_(PDLC).

[0098] Where the combined thickness of the PDLC plane and the polymerplane is substantially less than an optical wavelength (i.e.(t_(PDLC)+t_(p))<<λ), the grating will-exhibit form birefringence. Asdiscussed below, the magnitude of the shift in polarization isproportional to the length of the grating. Thus, by carefully selectingthe length, L, of the subwavelength grating for a given wavelength oflight, one can rotate the plane of polarization or create circularlypolarized light. Consequently, such subwavelength gratings can bedesigned to act as a half-wave or quarter-wave plate, respectively.Thus, an advantage of this process is that the birefringence of thematerial may be controlled by simple design parameters and optimized toa particular wavelength, rather than relying on the given birefringenceof any material at that wavelength.

[0099] To form a half-wave plate, the retardance of the subwavelengthgrating must be equal to one-half of a wavelength, i.e. retardance=λ/2,and to form a quarter-wave plate, the retardance must be equal toone-quarter of a wavelength, i.e. retardance=λ/4. It is known that theretardance is related to the net birefringence, |Δn|, which is thedifference between the ordinary index of refraction, n_(o), and theextraordinary index of refraction n_(e), of the subwavelength grating bythe following relation:

Retardance=|Δn|L=|n _(e) −n _(o) |L

[0100] Thus, for a half-wave plate, i.e. a retardance equal to one-halfof a wavelength, the length of the subwavelength grating should beselected so that:

L=λ/(2|Δn|)

[0101] Similarly, for a quarter-wave plate, i.e. a retardance equal toone-quarter of a wavelength, the length of the subwavelength gratingshould be selected so that:

L=λ/(4|Δn|)

[0102] If, for example, the polarization of the incident light is at anangle of 45° with respect to the optic axis 210 of a half-wave plate212, as shown in FIG. 14a, the plane polarization will be preserved, butthe polarization of the wave exiting the plate will be shifted by 90°.Thus, referring now to FIG. 14b and 14 c, where the half-wave plate 212is placed between cross polarizers 214 and 216, the incident light willbe transmitted. If an appropriate switching voltage is applied, as shownin FIG. 14d, the polarization of the light is not rotated and the lightwill be blocked by the second polarizer.

[0103] For a quarter wave plate plane polarized light is converted tocircularly polarized light. Thus, referring now to FIG. 15a, wherequarter wave plate 217 is placed between a polarizing beam splitter 218and a mirror 219, the reflected light will be reflected by the beamsplitter 218. If an appropriate switching voltage is applied, as shownin FIG. 15b, the reflected light will pass through the beam splitter andbe retroreflected on the incident beam.

[0104] Referring now to FIG. 16a, there is shown an elevational view ofa subwavelength grating 230 recorded in accordance with theabove-described methods and having periodic planes of polymer channels230 a and PDLC channels 230 b disposed perpendicular to the frontsurface 234 of grating 230. As shown FIG. 16a, the symmetry axis 232 ofthe liquid crystal domains is disposed in a direction parallel to thefront surface 234 of the grating and perpendicular to the periodicchannels 230 a and b of the grating 230. Thus, when an electric field Eis applied across the grating, as shown in FIG. 15b, the symmetry axis232 distorts and reorients in a direction along the field E, which isperpendicular to the front surface 234 of the grating and parallel tothe periodic channels 230 a and 230 b of the grating 230. As a result,subwavelength grating 230 can be switched between a state where itchanges the polarization of the incident radiation and a state in whichit does not. Without wishing to be bound by any theory, it is currentlybelieved that the direction of the liquid crystal domain symmetry 232 isdue to a surface tension gradient which occurs as a result of theanisotropic diffusion of monomer and liquid crystal during recording ofthe grating and that this gradient causes the liquid crystal domainsymmetry to orient in a direction perpendicular to the periodic planes.

[0105] As discussed is Born and Wolf, Principles of Optics, 5th Ed., NewYork (1975) and incorporated herein by reference, the birefringence of asubwavelength grating is given by the following relation:${n_{e}^{2} - n_{o}^{2}} = \frac{- \left\lbrack {\left( f_{PDLC} \right)\quad \left( f_{P} \right)\quad \left( {n_{PDLC}^{2} - n_{P}^{2}} \right)} \right\rbrack}{\left\lbrack {{f_{PDLC}n_{PDLC}^{2}} + {f_{P}n_{P}^{2}}} \right\rbrack}$

[0106] where

[0107] n_(o)=the ordinary index of refraction of the subwavelengthgrating;

[0108] n_(e)=the extraordinary index of refraction;

[0109] n_(PDLC)=the refractive index of the PDLC plane;

[0110] n_(p)=the refractive index of the polymer plane;

[0111] n_(LC)=the effective refractive index of the liquid crystal seenby an incident optical wave;

[0112] f_(PDLC)=T_(PDLC)/(t_(PDLC)+t_(p))

[0113] f_(p)=t_(p)/(t_(PDLC)+t_(p))

[0114] Thus, the net birefringence of the subwavelength grating will bezero if n_(PDLC)=n_(p).

[0115] It is known that the effective refractive index of the liquidcrystal, n_(LC), is a function of the applied electric field, having amaximum when the field is zero and a value equal to that of the polymer,n_(p), at some value of the electric field, E_(MAX). Thus, byapplication of an electric field, the refractive index of the liquidcrystal, n_(LC), and, hence, the refractive index of the PDLC plane canbe altered. Using the relationship set forth above, the netbirefringence of a subwavelength grating will be a minimum when n_(PDLC)is equal to n_(p), i.e. when n_(LC)=n_(p). Therefore, if the refractiveindex of the PDLC plane can be matched to the refractive index of thepolymer plane, i.e. n_(PDLC)=n_(p), by the application of an electricfield, the birefringence of the subwavelength grating can be switchedoff.

[0116] The following equation for net birefringence, i.e.|Δn|=|n_(e)−n_(o)|, follows from the equation given in Born and wolf(reproduced above):${\Delta \quad n} = \frac{- \left\lbrack {\left( f_{PDLC} \right)\quad \left( f_{P} \right)\quad \left( {n_{PDLC}^{2} - n_{P}^{2}} \right)} \right\rbrack}{\left\lbrack {2{n_{AVG}\left( {{f_{PDLC}n_{PDLC}^{2}} + {f_{P}n_{P}^{2)}}} \right\rbrack}} \right.}$

where n_(AVG)=(n_(e)+n_(o))/2

[0117] Furthermore, it is known that the refractive index of the PDLCplane n_(PDLC) is related to the effective refractive index of theliquid crystal seen by an incident optical wave, n_(LC), and therefractive index of the surrounding polymer plane, n_(p), by thefollowing relation:

n _(PDLC) =n _(p) +f _(LC) [n _(LC) −n _(p)]

[0118] where f_(LC) is the volume fraction of liquid crystal dispersedin the polymer within the PDLC plane, f_(LC)=[V_(LC)/(V_(LC)+V_(p))].

[0119] By way of example, a typical value for the effective refractiveindex for the liquid crystal in the absence of an electric field isn_(LC)=1.7, and for the polymer layer n_(p), =1.5. For a grating wherethe thickness of the PDLC planes and the polymer planes are equal (i.e.t_(PDLC)=t_(p), f_(PDLC)=0.5=f_(p)) and f_(LC)=0.35, the netbirefringence, Δn, of the subwavelength grating is approximately 0.008.Thus, where the incident light has a wavelength of 0.8 μm, the length ofthe subwavelength grating should be 50 μm for a half-wave plate and 25μm for a quarter-wave plate. Furthermore, by application of an electricfield of approximately 5 V/μm, the refractive index of the liquidcrystal can be matched to the refractive index of the polymer and thebirefringence of the subwavelength grating turned off. Thus, theswitching voltage, V_(n), for a half-wave plate is on the order of 250volts, and for a quarter-wave plate approximately 125 volts.

[0120] By applying such voltages, the plates can be switched between theon and off (zero retardance) states on the order of microseconds. As ameans of comparison, current Pockets cell technology can be switched innanoseconds with voltages of approximately 1000-2000 volts, and bulknematic liquid crystals can be switched on the order of millisecondswith voltages of approximately 5 volts.

[0121] In an alternative embodiment of the invention shown in FIG. 17,the switching voltage of the subwavelength grating can be reduced bystacking several subwavelength gratings 220 a-e together, and connectingthem electrically in parallel. By way of example, it has been found thata stack of five gratings each with a length of 10 μm yields thethickness required for a half-wave plate. It should be noted that thelength of the sample is somewhat greater than 50 μm, because eachgrating includes an indium-tin-oxide coating which acts as a transparentelectrode. The switching voltage for such a stack of plates, however, isonly 50 volts.

[0122] Subwavelength gratings in accordance with the present inventionare expected to find suitable application in the areas of polarizationoptics and optical switches for displays and laser optics, as well astunable filters for telecommunications, colorimetry, spectroscopy, laserprotection, and the like. Similarly, electrically switchabletransmission gratings have many applications for which beams of lightmust be deflected or holographic images switched. Among theseapplications are: Fiber optic switches, reprogrammable N×N opticalinterconnects for optical computing, beam steering for laser surgery,beam steering for laser radar, holographic image storage and retrieval,digital zoom optics (switchable holographic lenses), graphic arts andentertainment, and the like.

[0123] A switchable hologram is one for which the diffraction efficiencyof the hologram may be modulated by the application of an electricfield, and can be switched from a fully on state (high diffractionefficiency) to a fully off state (low or zero diffraction efficiency). Astatic hologram is one whose properties remain fixed independent of anapplied field. In accordance with the present invention, a high contraststatic hologram can also be created. In this variation of the presentinvention, the holograms are recorded as described previously. The curedpolymer film is then soaked in a suitable solvent at room temperaturefor a short duration and finally dried. For the liquid crystal E7,methanol has shown satisfactory application. Other potential solventsinclude alcohols each as ethanol, hydrocarbons such as hexane andheptane, and the like. When the material is dried, a high contraststatic hologram with high diffraction efficiency results. The highdiffraction efficiency is a consequence of the large index modulation inthe film (Δn˜0.5) because the second phase domains are replaced withempty (air) voids (n−1).

[0124] Similarly, in accordance with the present invention, a highbirefringence static subwavelength wave-plate can also be formed. Due tothe fact that the refractive index for air is significantly lower thanfor most liquid crystals, the corresponding thickness of the half-waveplate would be reduced accordingly. Synthesized wave-plates inaccordance with the present invention can be used in many applicationsemploying polarization optics, particularly where a material of theappropriate birefringence at the appropriate wavelength is unavailable,too costly, or too bulky.

[0125] In the claims, the term polymer-dispersed liquid crystals andpolymer-dispersed liquid crystal material includes, as may beappropriate, solutions in which none of the monomers have yetpolymerized or cured, solutions in which some polymerization hasoccurred, and solutions which have undergone complete polymerization.Those of skill in the art in the field of the invention will clearlyunderstand that the use herein of the standard term used in the art,polymer-dispersed liquid crystals (which grammatically refers to liquidcrystals dispersed in a fully polymerized matrix) is meant to includeall or part of a more grammatically correct prepolymer-dispersed liquidcrystal material or a more grammatically correct starting material for apolymer-dispersed liquid crystal material.

[0126] It will be seen that modifications to the invention as describedmay be made, as might occur to one with skill in the field of theinvention, within the intended scope of the claims. Therefore, allembodiments contemplated have not been shown in complete detail. Otherembodiments may be developed without departing from the spirit of theinvention or from the scope of the claims.

1. A slanted hologram made by exposing an interference pattern inside apolymer-dispersed liquid crystal material, the polymer-dispersed liquidcrystal material comprising, before exposure: (a) a polymerizablemonomer; (b) a liquid crystal; (c) a cross-linking monomer; (d) acoinitiator; and (e) a photoinitiator dye; wherein the hologram hasopposing surfaces and a plurality of polymer regions having a firstrefractive index and polymer-dispersed liquid crystal regions having asecond refractive index disposed at an angle to the opposing surfaces ofthe hologram whereby the symmetry axis of the liquid crystal is disposedat an angle to the opposing surfaces of the hologram.
 2. The slantedhologram of claim 1, wherein the polymerizable monomer comprisesdipentaerythritol hydroxypentaacrylate.
 3. The slanted hologram of claim1, wherein the polymer-dispersed liquid crystal material furthercomprises, before exposure, a surfactant.
 4. The slanted hologram ofclaim 1, wherein: (a) the liquid crystal comprises 10-40% by totalweight of the polymer-dispersed liquid crystal material; (b) thecross-linking monomer comprises 5-15% by total weight of thepolymer-dispersed liquid crystal material; (c) the amount of coinitiatoris 10⁻³ to 10⁻⁴ gram moles; and (d) the amount of photoinitiator dye is10⁻⁵ to 10⁻⁶ gram moles.
 5. The slanted hologram of claim 1, wherein:(a) the liquid crystal comprises 10-40% by total weight of thepolymer-dispersed liquid crystal material; (b) the cross-linking monomercomprises 10-18% by total weight of the polymer-dispersed liquid crystalmaterial; (c) the coinitiator comprises 2-3% by total weight of thepolymer-dispersed liquid crystal material; and (d) the photoinitiatordye comprises 0.2-0.4% by total weight of the polymer-dispersed liquidcrystal material.
 6. The slanted hologram of claim 2, wherein thesurfactant comprises about 6% by total weight of the polymer-dispersedliquid crystal material.
 7. The slanted hologram of claim 2, wherein thesurfactant comprises about 5-10% by total weight of thepolymer-dispersed liquid crystal material.
 8. The slanted hologram ofclaim 2, wherein the liquid crystal includes a mixture of cyanobiphenyls.
 9. The slanted hologram of claim 1, wherein the cross-linkingmonomer comprises N-vinylpyrrolidone.
 10. The slanted hologram of claim1, wherein the coinitiator comprises N-phenylglycine.
 11. The slantedhologram of claim 1, wherein the photoinitiator dye comprises rosebengal ester.
 12. The slanted hologram of claim 3, wherein thesurfactant comprises octanoic acid.
 13. The slanted hologram of claim 4,wherein the polymer-dispersed liquid crystal material further comprises,before exposure, a surfactant.
 14. The slanted hologram of claim 5,wherein the polymer-dispersed liquid crystal material further comprises,before exposure, a surfactant.
 15. The slanted hologram of claim 4,wherein the polymerizable monomer comprises dipentarythritolhydroxypentaacrylate.
 16. The slanted hologram of claim 5, wherein thepolymerizable monomer comprises dipentarythritol hydroxypentaacrylate.17. A slanted hologram made by exposing an interference pattern inside apolymer-dispersed liquid crystal material, the polymer-dispersed liquidcrystal material comprising, before exposure: (a) a polymerizablemonomer; (b) a liquid crystal; (c) a cross-linking monomer; (d) acoinitiator; and (e) a photoinitiator dye; wherein the hologram hasopposing surfaces and a plurality of alternative planes of polymerchannels having a first refractive index and polymer-dispersed liquidcrystal channels having a second refractive index disposed at an angleto the opposing surfaces of the hologram whereby the symmetry axis ofthe liquid crystal is disposed at an angle to the opposing surfaces ofthe hologram.
 18. The slanted hologram of claim 17, wherein thepolymerizable monomer comprises dipentarythritol hydroxypentaacrylate.19. The slanted hologram of claim 17, wherein the polymer-dispersedliquid crystal material further comprises, before exposure, asurfactant.
 20. The slanted hologram of claim 17, wherein: (a) theliquid crystal comprises 10-40% by total weight of the polymer-dispersedliquid crystal material; (b) the cross-linking monomer comprises 5-15%by total weight of the polymer-dispersed liquid crystal material; (c)the amount of coinitiator is 10⁻³ to 10⁻⁴ gram moles; and (d) the amountof photoinitiator dye is 10⁻⁵ to 10⁻⁶ gram moles.
 21. The slantedhologram of claim 17, wherein: (a) the liquid crystal comprises 10-40%by total weight of the polymer-dispersed liquid crystal material; (b)the cross-linking monomer comprises 10-18% by total weight of thepolymer-dispersed liquid crystal material; (c) the coinitiator comprises2-3% by total weight of the polymer-dispersed liquid crystal material;and (d) the photoinitiator dye comprises 0.2-0.4% by total weight of thepolymer-dispersed liquid crystal material.
 22. The slanted hologram ofclaim 18, wherein the surfactant comprises about 6% by total weight ofthe polymer-dispersed liquid crystal material.
 23. The slanted hologramof claim 18, wherein the surfactant comprises about 5-10% by totalweight of the polymer-dispersed liquid crystal material.
 24. The slantedhologram of claim 18, wherein the liquid crystal includes a mixture ofcyano biphenyls.
 25. The slanted hologram of claim 17, wherein the crosslinking monomer comprises N-vinylpyrrolidone.
 26. The slanted hologramof claim 17, wherein the coinitiator comprises N-phenylglycine.
 27. Theslanted hologram of claim 17, wherein the photoinitiator dye comprisesrose bengal ester.
 28. The slanted hologram of claim 19, wherein thesurfactant comprises octanoic acid.
 29. The slanted hologram of claim20, wherein the polymer-dispersed liquid crystal material furthercomprises, before exposure, a surfactant.
 30. The slanted hologram ofclaim 21, wherein the polymer-dispersed liquid crystal material furthercomprises, before exposure, a surfactant.
 31. The slanted hologram ofclaim 20, wherein the polymerizable monomer comprises dipentarythritolhydroxypentaacrylate.
 32. The slanted hologram of claim 21, wherein thepolymerizable monomer comprises dipentarythritol hydroxypentaacrylate.33. A static hologram made by exposing an interference pattern inside apolymer-dispersed liquid crystal material, the polymer-dispersed liquidcrystal material comprising, before exposure: (a) a polymerizablemonomer; (b) a liquid crystal; (c) a cross-linking monomer; (d) acoinitiator; and (e) a photoinitiator dye; wherein the hologram has aplurality of polymer regions having a first refractive index andpolymer-dispersed liquid crystal regions having a second refractiveindex wherein at least a portion of the liquid crystal in thepolymer-dispersed liquid crystal regions has been removed.
 34. A statichologram made by exposing an interference pattern inside apolymer-dispersed liquid crystal material, the polymer-dispersed liquidcrystal material comprising, before exposure: (a) a polymerizablemonomer; (b) a liquid crystal; (c) a cross-linking monomer; (d) acoinitiator; and (e) a photoinitiator dye; wherein the hologram has aplurality of alternating planes of polymer channels having a firstrefractive index and polymer-dispersed liquid crystal channels having asecond refractive index wherein at least a portion of the liquid crystalin the polymer-dispersed liquid crystal channels has been removed.
 35. Amethod for preparing static hologram, comprising: disposing apolymer-dispersed liquid crystal material between transparent plates;exposing an interference pattern inside the polymer-dispersed liquidcrystal material thereby forming a hologram having polymer regionshaving a first refractive index and polymer-dispersed liquid crystalregions having a second refractive index; and removing at least aportion of the liquid crystal in the polymer-dispersed liquid crystalchannels.
 36. A method for preparing a static hologram, comprising:disposing a polymer-dispersed liquid crystal material betweentransparent plates; exposing an interference pattern inside thepolymerdispersed liquid crystal material thereby forming a hologramhaving alternating planes of polymer channels having a first refractiveindex and polymer-dispersed liquid crystal channels having a secondrefractive index; and removing at least a portion of the liquid crystalin the polymer-dispersed liquid crystal channels.
 37. The method ofclaims 35 or 36, wherein a portion of the liquid crystal in thepolymer-dispersed liquid crystal channels is removed by disposing thehologram in a solvent.